The present invention relates to a cyclopentadienyliron complex salt, and more particularly to an (.eta.-ketoarene)-(.eta.-cyclopentadienyl)iron(II) salt useful as a photoinitiator. The present invention also relates to a process for preparing the complex salt and a photopolymerizable composition containing it as a self sensitized initiator.
Since the first reported synthesis of a metal sandwich compound almost forty years ago [T. J. Keally and P. L. Pauson, Nature 1951, 168, 1039], there has been significant interest in the chemistry and properties of these transition metal complexes. Transition metal complexes of the type (arene).sub.2 M.sup.+n, where arene is a charged or uncharged aromatic species and M.sup.+n is a transition metal such as Cr.sup.+2 or Fe.sup.+2, are known to activate the arene(s) to nucleophilic substitution and increase the acidity of .alpha. alkyl protons. Mixed arene metal sandwiches of the type Cp-Fe(II)-arene, where Cp is deprotonated cyclopentadiene and the arene is a neutral aromatic compound such as mesitylene, have been extensively studied [D. Astruc, Tetrahedron 1983, 39(24), 4027].
There are two important methods for the production of Cp-Fe(II)-arene complexes. The first method involves exchanging the carbonyls in dicarbonyl-.eta..sup.5 -(2,4-cyclopentadien-1-yl)iron(II) halide for an arene under the influence of a Lewis acid such as aluminum chloride [T. H. Coffield, V. Sandel, R. D. Closson, J. Am. Chem. Soc., 1957, 79, 5826]. This method yields complexes in fair yield (40%) but suffers from the production of various side products lowering overall yield. By far the most useful method for the production of these compounds is to exchange the cyclopentadienyl ligand of ferrocene for the arene in the presence of aluminum chloride and aluminum dust [A. N. Nesmeyanov, N. A. Vol'kenau, E. L. Sirotkina and L. S. Shilovseva, Kokl. Akad. Nauk, SSSR 1965, 166, 607]. This second method has become the standard method for the production of these complexes since the starting material is readily available and yields are better.
For sandwich complexes of this type, the functionalization of the neutral aromatic compound has been rather limited. There are numerous examples of electron donor substituted aromatic exchange reactions and relatively few examples of exchange reactions involving electron acceptor substituted aromatic compounds [D. Astruc, Tetrahedron 1983, 39(24), 4027]. Some examples of electron acceptor substituted arenes include the halogenated aromatics and exocyclic heteroatom aromatics such as aniline. Although these examples are well known, the yields are generally low even under the most drastic conditions. For example, the degree of halogenation significantly influences the yield so that mono and dihalogenated arene complexes can be obtained but trihalogenated arene complexes are not known. In the case of exocyclic heteroatom aromatics, the conditions must be drastic and the yields are often low [J. F. Helling and W. A. Hendrickson, J. Organomet. Chem. 1979, 168, 87]. Electron acceptor groups on the arene inhibit or prevent ligand exchange. Additionally, the requirement of a rather strong Lewis acid to catalyze the ligand exchange limits the possible candidate aromatic ligand to one which is stable under these conditions. To date there has been no example of a direct exchange of a keto arene, perhaps as a result of the electron withdrawing nature of the conjugated carbonyl group or the formation of a complex with the aluminum chloride.
It is reported that chlorobenzene-Fe(II)-Cp is converted to acetophenone-Fe(II)-Cp via substitution of the chlorine on the arene with ethyl nitrite followed by a Nef type hydrolysis of the intermediate nitro species by hydrochloric acid (U. S. Gill, Inorg. Chem. Acta, 1986, 114, L25). However, yields for this reported reaction are highly variable and the optimal conditions are not known. A simpler method to produce a keto substituted arene complex is highly desirable.
European Patent No. 126,712 discloses that various arene metal sandwich complexes are useful as photoinitiators for cationic polymerization of monomers such as epoxides, representative examples of which are Cp-Fe(II)-arene wherein the arene is benzene, toluene, cumene, naphthalene, anthracene, phenanthrene or pyrene. This type of compound is photolyzed by near UV or visible radiation with release of arene to form a Cp-Fe(II) ion. It is important that the UV energy absorption of the arene complex at 365 nm is sufficient to initiate a polymerization. The absorptivity of the proposed Cp-Fe(II)-arene initiators increases with increasing the numbers of benzene rings. Anthracene-Fe(II)-Cp and pyrene-Fe(II)-Cp complex salts are usable as self sensitized initiators. However, these initiators may impair environmental safety due to release of harmful arene such as pyrene into the environment. The UV energy absorptivity of benzene-Fe(II)-Cp and cumene-Fe(II)-Cp is not sufficient initiate a polymerization at an acceptable speed. European Patent No. 126,718 proposes to use a cumene-Fe(II)-Cp initiator in the presence of a peroxide and an external sensitizer.
It is an object of the present invention to provide a novel (keto substituted .eta..sup.6 -arene)-(.eta..sup.5 -cyclopentadienyl)iron(II) complex salt.
A further object of the present invention is to provide a (keto substituted .eta..sup.6 -arene)-(.eta..sup.5 -cyclopentadienyl)iron(II) complex salt which is useful as a self sensitized photoinitiator for cationically polymerizable monomers, especially for epoxide compounds.
A still further object of the present invention is to provide a photoinitiating system which can initiate polymerization with a minimized release of arene during or after polymerization and which applicable to a polymerization system containing both a cationically polymerizable monomer and a radically polymerizable monomer without requiring any presence of a peroxide and a sensitizer.
Another object of the present invention is to provide a keto substituted arene complex as an intermediate to produce an epoxidized arene complex by a known method.
Still another object of the present invention is to provide a simple process for preparing a keto substituted arene metal complex by a direct replacement of a keto arene.
These and other objects of the present invention will become apparent from the description hereinafter.